Projection-type display apparatus for projecting an image onto a screen have heretofore often employed a high-pressure mercury lamp as a light source. In recent years, attention has been paid to projection-type display apparatus employing LEDs instead of high-pressure mercury lamps.
LEDs are advantageous in that their brightness can be adjusted more quickly than high-pressure mercury lamps. Therefore, projection-type display apparatus which employ LEDs as a light source are capable of adjusting the brightness of a projected image depending on the brightness of the surrounding area by adjusting the brightness of the LEDs depending on the brightness of the surrounding area. Since the LEDs can be prevented from being energized for a higher level of brightness than necessary, the power consumption of the projection-type display apparatus is reduced.
The LEDs have a low power conversion efficiency (WPE: Wall Plug Efficiency), which represents the ratio of light output power to applied electric power, in the range from about 5% to 15%. Much of the applied electric power is converted into heat, which tends to increase the junction temperature Tj of the LEDs. If the junction temperature Tj of the LEDs goes higher than a certain level, then the light output power of the LEDs is abruptly reduced. Consequently, the projection-type display apparatuses which employ LEDs as a light source usually cool the LEDs so that the junction temperature Tj of the LEDs will not exceed a maximum junction temperature Tj(max). The LEDs are often cooled by a cooling means such as a Peltier device that is able to adjust the cooling power depending on the amount of applied electric power (see Patent document 1).
FIG. 1 is a block diagram showing the configuration of a projection-type display apparatus having a cooling means for cooling LEDs and capable of changing the brightness of the LEDs depending on the brightness of the surrounding area.
As shown in FIG. 1, projection-type display apparatus 200 includes light sources 204R, 204G, 204B for emitting a red beam, a green beam, and a blue beam, respectively, coolers 205R, 205G, 205B for cooling respective light sources 204R, 204G, 204B, and adjuster 201 for adjusting the amount of electric power applied to light sources 204R, 204G, 204B and the amount of electric power applied to coolers 205R, 205G, 205B.
Each of light sources 204R, 204G, 204B comprises an LED and an LED case housing the LED. Each of coolers 205R, 205G, 205B comprises a Peltier device.
Adjuster 201 adjusts the brightness of each LED by adjusting the amount of applied light source electric power W, which is the amount of electric power applied to the LED, depending on the detection result of light sensor 202 that detects the brightness of the surrounding area.
Adjuster 201 also adjusts the amount of applied cooler electric power, which is the amount of electric power applier to coolers 205R, 205G, 205B, depending on the detection results of light source case temperature detectors 210R, 210G, 210B that detect case temperatures Tc of the LED cases of respective light sources 204R, 204G, 204B.
Generally, it is known that the equation “Tj=If×Vf×(1−WPE/100)×Rj−c+Tc” is satisfied between the junction temperature Tj of the LED and the case temperature Tc of the LED case where if represents a forward current of the LED, Vf a forward voltage of the LED, WPE, a power conversion efficiency of the LED, and Rj−c the thermal resistance between the LED junction and the LED case. In the above equation, the product If×Vf of the forward current If and the forward voltage Vf represents the amount of applied power supply electric power W that is applied to the LED. The power conversion efficiency WPE and the thermal resistance Rj−c are usually regarded as constants that are determined for each LED.
As can be seen from the above equation, if the amount of applied power supply electric power W is constant, then the maximum case temperature which is the case temperature Tc at the time the junction temperature Tj is the maximum junction temperature Tj(max) is uniquely determined. If the amount of applied cooler electric power is adjusted so that the case temperature Tc becomes the maximum case temperature, then the junction temperature Tj will not exceed the maximum junction temperature Tj(max).
However, with the projection-type display apparatus 200, since the amount of applied power supply electric power W changes depending on the brightness of the surrounding area, the maximum case temperature also changes. More specifically, as the amount of applied power supply electric power W becomes greater, the maximum case temperature goes lower. In order that the junction temperature Tj will not exceed the maximum junction temperature Tj(max) even if the maximum case temperature changes, adjuster 201 adjusts the amount of applied cooler electric power so that the case temperature Tc is equalized to the maximum case temperature when the amount of applied power supply electric power W is of an upper limit value.